The heat low over Pakistan/northwestern India: spatiotemporal evolution and forcing mechanisms

A thermal (or heat) low is present from May to September over the arid areas of Pakistan and western India, as part of the low-pressure belt extending over the deserts from the Sahara to India across the Middle East. The heat low is an integral component of the South Asian summer monsoon but its origin remains enigmatic, in part because the low is deeper in July than in May (when it is warmer) and because of the paucity of in-situ observational data.

The emphasis of this work is on clarifying the physical processes responsible for the development and maintenance of the heat low (from late-spring to summer) and its relationship with the large-scale circulation. Results from observational and a two-level modeling studies will be presented.

The three-dimensional climatological features of the low are characterized by means of high-resolution reanalysis data. Insight into how the low is influenced by the large-scale circulation is provided, as well as of its links with the ensuing South Asian monsoon. In particular, the mature phase of the low is shown to be remotely modulated by intense diabatic heating over the eastern Indian Ocean associated with the summer monsoon.

The dynamical forcing of surrounding mountains on the low (i.e., its localization and intensification) is investigated by means of sensitivity experiments with a linear primitive equation model. Regional orography is shown to be important for the strengthening of the low as the season progresses.

The influence of regional surface properties (e.g., surface albedo, which affects the local thermal forcing) and orography (i.e., the surrounding mountains – the Zagros Plateau and the western Himalayas) is also investigated through high-resolution climate simulations with the latest version of the Weather Research and Forecasting model (WRF). The results of these sensitivity simulations will be discussed. In particular, the interaction of the large-scale flow with the mountains is shown to modify the thermodynamic balance of the region and to generate subsidence over the low during June and July, positively contributing to the maintenance of the low itself.